Net::Netmask parses and understands IPv4 CIDR blocks.  It's built with
an object-oriented interface and provides nearly all bits of information
about a network block that you might want.

There are also functions to put a network block into a table and look it
up by IP address, turn an IP address range into a list of CIDR blocks,
turn a list of CIDR blocks into a list of IP addresses, and a function
for sorting by text IP address.
